1. Field of the Invention
The present invention relates to a fuel cell system, and more particularly, to a fuel cell system having a structure capable of interrupting a supply of a fuel when a fuel cell or an electronic device connected thereto heats up to an abnormally high temperature.
2. Description of the Related Art
The fuel cell converts chemical energy obtained by chemically reacting a fuel, such as hydrogen, with oxygen directly into electric energy.
With this fuel cell, the energy capacity per volume/per weight can be dramatically increased compared to a related-art battery because the energy density of the fuel is high and there is no need for an active material on a cathode side due to oxygen being supplied by outside air.
Among various fuel cells, a polymer electrolyte fuel cell (PEFC) has a full solid structure using a flexible polymer film as an electrolyte. As such, the fuel cell is easy to handle. Also, this fuel cell has a simple structure, can be operated at low temperature, and can be activated an and deactivated in a short period of time.
Therefore, it can be said that the polymer electrolyte fuel cell is suitable for being mounted on mobile electronic devices.
The polymer electrolyte fuel cell basically includes a polymer electrolyte membrane having proton conductivity and a pair of electrodes provided at both surfaces of the polymer electrolyte membrane.
The electrodes each include a catalyst layer made of platinum or a platinum group metal catalyst and a gas diffusion electrode formed on the outside of the catalyst layer for supplying a gas and collecting current.
An assembly in which the electrodes and the polymer electrolyte membrane are integrated into one is referred to as a membrane electrode assembly (MEA) having such a structure that a fuel (hydrogen) is supplied to one of the electrodes and an oxidizer (oxygen) is supplied to another electrode to generate power.
A theoretical voltage of a fuel cell unit including a pair of membrane electrode assemblies is about 1.23 V. In a normal operational state, the fuel cell unit is driven by about 0.7 V in many cases.
Accordingly, in the case where a higher activation voltage is required, a plurality of fuel cell units are laminated and each fuel cell unit is arranged electrically in series to be used.
This type of a laminate structure is called a fuel cell stack. In the stack, normally, an oxidizer flow path and a fuel flow path are isolated by a member called as a separator.
There are various kinds of methods of supplying fuel to the fuel cell. For example, the methods of supplying the fuel include: a method involving directly supplying a liquid fuel, such as methanol; a method involving supplying pure hydrogen; and a method involving modifying liquid fuel to generate hydrogen and supplying the hydrogen to a fuel electrode.
The hydrogen supply method can be used for mobile electronic devices because it leads to a high output and is advantageous with respect to downsizing.
In an operation of the fuel cell system, temperature control is important.
When a temperature of a power generation portion of the fuel cell exceeds an allowable range and becomes too high at a time of driving, the polymer electrolyte membrane is dried and proton conductivity decreases. Due to the occurrence of a so-called dryout, electric characteristics are reduced.
When a high load is applied in a dryout state, a power generation capacity of the fuel cell cannot keep up with this load, leading to a large polarity reversal and thereby seriously damaging the membrane electrode assembly (MEA) in some cases.
Further, when the fuel cell system is kept at a high temperature, a deterioration of components of the fuel cell may occur.
Further, other than in a normal operation, when the fuel cell is externally short-circuited, or when catalytic combustion is caused due to the damage of the membrane, the temperature of the power generation portion of the fuel cell exhibits a severe increase exceeding the allowable temperature range.
In this state, the part of the electronic device to which the fuel cell is mounted may be damaged.
Therefore, a measure for stopping the power generation is necessary when an abnormally high temperature exceeding the allowable range (hereinafter, merely referred to as “abnormally high temperature”) is reached.
A method of interrupting the supply of the fuel is a reliable way of stopping the power generation when the fuel cell is at an abnormally high temperature. Conventionally, a method of this type is suggested.
For example, Japanese Patent Application Laid-Open No. H09-147895 discloses a method in which a temperature sensor and a control device are mounted, and when the abnormally high temperature is reached, a fuel supply valve is closed by the control device.
Further, Japanese Patent Application Laid-Open. No. 2001-229942 discloses a method in which a material that deforms at a high temperature is disposed in the fuel flow path. When the abnormally high temperature is reached, the fuel flow path is interrupted by the deformation of that material.
However, in Japanese Patent Application Laid-Open No. H09-147895, there is a problem in that an external power source is required in order to detect the temperature using a sensor and to operate the control device. Also, active control using the control device is required. This results in an increase in the size of the system.
Further, Japanese Patent Application Laid-Open No. 2001-229942 has a problem in that the interruption of the supply of highly dispersible hydrogen in the fuel flow path is unreliable. Also, the ability of the deformed material to return to its previous shape is doubtful.